1. Technical Field
The present disclosure relates to a duplex control device and a duplex system.
2. Description of the Related Art
As one of the techniques for enhancing reliability of a device, a duplex method has been employed. In a duplex system, one system is set as a service side for executing practical operations, while the other system is set as a stand-by side. If a failure is found in the service side, the system that is in charge of the practical operations is switched to the stand-by side.
FIG. 6 illustrates an example of a configuration of a typical duplex system 300. In this example, a current output device for outputting a 4-mA to 20-mA current corresponding to a manipulated value MV is duplexed. The duplexing in this example is realized by a current output device A 310a and a current output device B 310b. The current output device A 310a and the current output device B 310b have an identical configuration, and receive an identical manipulated value MV. Thus, the current output device B 310b will be described in a simplified manner.
The duplex system 300 includes a duplex controller A 320a, which monitors operation of the current output device A 310a, and a duplex controller B 320b, which monitors operation of the current output device B 310b. Both of the duplex controller A 320a and the duplex controller B 320b are connected to a higher level system that controls the duplex system 300.
The duplex controller A 320a monitors operation of the current output device A 310a. When the current output device A 310a is normal, the duplex controller A 320a sets a level of a ReqA signal, which requests a control right, at H. When abnormality in the current output device A 310a is detected, the duplex controller A 320a sets the level of the ReqA signal at L. The duplex controller B 320b monitors operation of the current output device B 310b. When the current output device B 310b is normal, the duplex controller B 320b sets a level of a ReqB signal, which requests the control right, at H. When abnormality in the current output device B 310b is detected, the duplex controller B 320b sets the level of the ReqB signal at L.
The duplex system 300 includes a switching device 330. The switching device 330 switches, between the current output device A 310a and the current output device B 310b, a current output device to serve as the service side (or as the stand-by side). The switching device 330 receives the ReqA signal and the ReqB signal. Further, the switching device 330 outputs a ServiceA signal to the current output device A 310a, and outputs a ServiceB signal to the current output device B 310b. 
The switching device 330 may include an SR latch circuit. In the example shown in FIG. 6, the switching device 330 includes a negative logic SR latch circuit that includes a NAND gate G1 and a NAND gate G2.
Namely, the NAND gate G1 receives the ReqA signal and the ServiceB signal that is an output from the NAND gate G2, and outputs the ServiceA signal. The NAND gate G2 receives the ReqB signal and the ServiceA signal that is an output from the NAND gate G1, and outputs the ServiceB signal.
A path for feeding back, to the NAND gate G2, the ServiceA signal that is output from the NAND gate G1 is referred to as “fbA.” A path for feeding back, to the NAND gate G1, the ServiceB signal that is output from the NAND gate G2 is referred to as “fbB.”
In the current output device A 310a, when the level of the ServiceA signal is L, a transistor Tr1 is off. Therefore, in this state, the current output device A 310a serves as the service side for outputting a 4-mA to 20-mA current corresponding to a manipulated value MV. Meanwhile, when the level of the ServiceA signal is H, the transistor Tr1 is on. Therefore, in this state, the current output device A 310a does not output the current, and serves as the stand-by side. When the level of the ServiceB signal is L, the current output device B 310b serves as the service side. Meanwhile, when the level of the ServiceB signal is H, the current output device B 310b serves as the stand-by side.
When both of the current output device A 310a and the current output device B 310b are normal, one of the duplex controller A 320a and the duplex controller B 320b requests the control right ahead of the other, under control of the higher level system. When the duplex controller A 320a requests the control right ahead of the duplex controller B 320b, the level of the ReqA signal becomes H and the level of the ReqB signal becomes L.
This allows the level of the ServiceA signal to be at L stably, and allows the level of the ServiceB signal to be at H stably. As a result, the current output device A 310a serves as the service side, and the current output device B 310b serves as the stand-by side.
If the duplex controller B 320b requests the control right thereafter, the levels of both of the ReqA signal and the ReqB signal become H. In this state, the output state of the switching device 330 is maintained. Thus, the current output device A 310a continues serving as the service side, and the current output device B 310b continues serving as the stand-by side.
If the duplex controller A 320a, which is in the service side, detects abnormality in the current output device A 310a while the duplex system 300 is operating, the duplex controller A 320a abandons the control right and sets the level of the ReqA signal at L. Consequently, the level of the ServiceA signal is inverted to H, and the level of the ServiceB signal is inverted to L. As a result, the current output device B 310b serves as the service side, and the current output device A 310a serves as the stand-by side. This allows the duplex system 300 to continue operating without stopping the service.
Further, the duplex controller A 320a notifies the abnormality in the current output device A 310a to the higher level system. Consequently, while the current output device B 310b serves as the service side, the current output device A 310a can be repaired or exchanged. Further, after that, the duplex controller A 320a can request the control right again. This allows the current output device A 310a to be immediately switched to the service side even if abnormality in the current output device B 310b is detected thereafter. This allows the duplex system 300 to continue operating without stopping the service.
On the other hand, if the duplex controller B 320b, which is in the stand-by side, detects abnormality in the current output device B 310b, the duplex controller B 320b sets the level of the ReqB signal at L. In this state, the output state of the switching device 330 is maintained. This allows the current output device A 310a to continue working as the service side.
Further, the duplex controller B 320b notifies the abnormality in the current output device B 310b to the higher level system. Consequently, while the current output device A 310a serves as the service side, the current output device B 310b can be repaired or exchanged. Further, after that, the duplex controller B 320b can request the control right again. This allows the current output device B 310b to be immediately switched to the service side even if abnormality in the current output device A 310a is detected thereafter. This allows the duplex system 300 to continue operating without stopping the service.
Examples of the documents related to such a technique encompass JP-A-2000-222001.